1. Field of the Invention
The present invention relates generally to a piezoelectric transformer, and more specifically to a voltage step-up type piezoelectric transformer which features an improved electrode arrangement which reduces the size of the transformer and ensures highly reliable operation thereof.
2. Description of Related Art
In most of cases, an electromagnetic type transformer is used for generating a high output voltage, of the nature used in photocopiers, backlights of LCD (Liquid Crystal Displays) or the like.
On the other hand, a piezoelectric transformer which operates on a principle which is totally different from that of a conventional electromagnetic transformer, has been proposed.
Throughout the instant disclosure, like portions or elements of the drawings are denoted by like reference numerals.
Before turning to the present invention it is deemed preferable to describe a conventional voltage step-up type piezoelectric transformer with reference to FIG. 1.
The piezoelectric transformer as shown in FIG. 1 is disclosed in U.S. Pat. No. 2,974,296 granted to C. A. Rosen.
As shown in FIG. 1, a long relatively thin rectangular body 10 of piezoelectric material is divided into two regions 12 and 14 each of which extends in a longitudinal direction of the body 10 a distance equal to one half of the length thereof. The region 12 is referred to as a driving region while the region 14 is referred to as a driven region (or power generating region).
A pair of electrodes 16a and 16b is applied to upper and lower surfaces of the driving region 12. On the other hand, the driven region 14 has an electrode 18 applied to the end of the body 10 remote from the region 12. The driving region 12 is polarized in the thickness direction of the body 10 as indicated by arrows 13 while the driven region 14 is polarized in the longitudinal direction of the body 10 as indicated by arrows 15.
A connection wire 20a has one end thereof fixedly coupled to a junction 21 provided at the center of the electrode 16a. Similarly, a connection wire 20b has one end thereof fixedly coupled to a junction (not shown) provided at the center of the electrode 16b. On the other hand, a connection wire 24a is provided between a junction 25 and an output terminal 26a. Further, a connection wire 24b has one end thereof coupled to an output terminal 26b and the other end thereof coupled to the wire 20b.
If an input voltage Vin applied to the pair of input terminals 22a and 22b has a frequency equal to that of the longitudinal resonance vibration of the body 10, the body 10 is caused to resonantly vibrate. Thus, a high AC (alternating current) voltage Vout is obtained between the output terminals 26a and 26b. The frequency of the output voltage Vout is identical to that of the input voltage Vin.
The above-mentioned piezoelectric transformer, however, has encountered the following drawbacks.
In the case where the piezoelectric transformer shown in FIG. 1 is utilized as a voltage step-up transformer, a voltage transformation ratio (denoted by Rv) under ideal conditions is given by EQU Rv.varies.L/2d.K (1)
where L is the length of the body 10 in the longitudinal direction thereof;
d is the thickness of the body 10; and PA1 K is a constant (usually less than unity) depending on compliance and electro-mechanical coupling coefficients of the material of the body 10 when taking anisotropy into account.
Expression (1) indicates that the transformation ratio Rv is proportional to L/2d.
The body 10 is typically manufactured as follows. Initially, a piezoelectric ceramic block is cut into a piece having a suitable size and then ground to the required thickness. However, it is practically difficult to reduce the thickness of the body 10 to an extent ranging less 0.2 mm or 0.3 mm. Accordingly, if the voltage transformation ratio Rv of about 100 is required, the length L of the body 10 should be more than 30 mm. Thus, the length of the driven region 14 is 15 mm (viz., 30 mm/2).
This means that the driven region 14 should be polarized in the longitudinal direction thereof over the full length thereof (viz., 15 mm). In such a case, DC (direct current) voltage as high as 50-60 KV is required for the polarization at a room temperature. Thus, the above-mentioned prior art has encountered the drawback that an expensive high voltage power source is undesirably required. Further, such a high DC voltage is liable to induce dielectric breakdown between the polarizing electrodes during polarization, and hence it is in practice extremely difficult to polarize such a lengthy region.
One approach to overcoming the above-mentioned problems is to polarize the body 10 while maintaining the temperature thereof over a Curie temperature (about 300.degree. C.). In this instance, a voltage of about 5-6 KV is still required for polarizing the driven region 14 whose longitudinal length is about 15 mm. Additionally, another problem has been encountered in this case in that it is very difficult to electrically isolate the electrodes 16a-16b and 18 at high temperatures such these in excess of 300.degree. C.
Accordingly, with the arrangement of FIG. 1, it is practically impossible to achieve a sufficiently high voltage transformation ratio.
On the other hand, it is known that the transformation ratio can be elevated with increase in the lateral cross section of the body 10. That is, if the width of the body is increased, the transformation ratio can be raised. However, as the width increases relative to the longitudinal length of the body, spurious vibrations (viz., vibrations in the transverse direction of the body) become noticeable to the degree that longitudinal vibrations (d) of the body 10 become undesirably weakened. Thus, the width of the body should be less than about one-fourth (1/4) of the longitudinal length (L).
From the foregoing, it will be understood that the arrangement shown in FIG. 1 is not expected to achieve a high voltage step-up transformation ratio.